In wideband wireless communication systems, the signal often tends to weaken from frequency selective fading due to multi-path transmissions. Frequency selective fading is a radio propagation anomaly generally caused by the partial cancellation of a radio signal by itself As the signal arrives at the receiver by multiple different paths, and at least one of the paths is changing (lengthening or shortening), the combination of the multiple signals sometimes causes partial signal cancellations.
Orthogonal frequency division multiplexing (OFDM) systems have been proposed to overcome the problem of frequency selective fading by dividing the total bandwidth into multiple subcarriers, such that the bandwidth on each subcarrier is sufficiently narrow to enable the data modulation symbols carried by that subcarrier to experience relatively flat fading. An OFDMA system uses the OFDM modulation technique to multiplex the data traffic of several mobile stations in both frequency and time.
FIG. 1 is a block diagram illustrating a typical example of framing structure 10 in an OFDM or OFDMA-based wireless communications system. Communication stream 100 includes the stream of frames that make up the communication transmission. Communication stream 100 typically has multiple preamble frames, such as preamble frame 101, which delimit a particular number, i.e., M, of traffic frames, such as M traffic frames 102-103. The unit made up of the preamble and traffic frames is known as a superframe, such as superframe 111.
Superframe 111 is made up from preamble frame 101 and traffic frames 102 through 103. In an OFDM system, preamble frame 101 and traffic frame 102 consists of multiple OFDM symbols. For example, traffic frame 103 contains OFDM symbol 1-104, OFDM symbol 2-105, through OFDM symbol N-106. Each OFDM symbol, such as OFDM symbol 105, includes inverse fast Fourier transform (IFFT) symbol 109, which is the result of an IFFT operation on the modulation data sequence, cyclic prefix (CP) 108, which is a copy of the last portion of IFFT symbol 109 and is inserted before the IFFT symbol 109, and windowing periods 107 and 110, which shape the modulation pulse so that the radio spectrum of the transmitted signal meets the emission mask requirement set forth by the radio regulatory body, such as the Federal Communication Commission (FCC) in the United States.
Preamble 101 of superframe 111 provides control information for a mobile station to acquire the base station signals in the power-up procedure or to continue to receive the signaling of the updated system parameters after the mobile station becomes active in the system.
FIG. 2 is a diagram illustrating exemplary OFDM preamble structure 200 proposed for the air interface evolution (AIE) of cdma2000 standards. Preamble 200 comprises eight OFDM symbols, including, in the order in which each is transmitted: one OFDM symbol for the primary broadcast control channel (PBCCH), PBCCH symbol 201, which includes the information of the number of guard tones used in the system; four OFDM symbols, SBCCH/QPCH symbols 202, which comprise the secondary broadcast control channels (SBCCHs) in the even-numbered superframes and comprise the quick paging channels (QPCHs) in the odd-numbered superframes; one OFDM symbol for the acquisition pilot, TDM1 203, that is used by the mobile station to acquire: (1) the superframe and the OFDM symbol timing, (2) the size of the fast Fourier transform (FFT) used on the superframe preamble, and (3) the length of the CP used in the system; one OFDM symbol for the acquisition pilot, TDM2 204, that carries 9-bit sector identity information, known as PilotPN, in asynchronous systems, or carries 9-bit PilotPhase in synchronous systems; and one OFDM symbol for the acquisition pilot, TDM3 205, that carries additional 9-bit system parameters. The 9-bit Pilot PN and PilotPhase information carried by TDM2 204 is generally used to facilitate signal processing gain across different superframe preambles, where the PilotPhase is typically equal to PilotPN+ system time, where system time is the superframe index.
First, preamble 200 provides a timing reference for the mobile station receiver to align with each received OFDM symbol and to correctly remove CP 108 (FIG. 1) before decoding the data. This is accomplished by having a sequence repeated once in the acquisition pilot TDM1 203. The mobile station receiver detects the timing by constantly searching the peak of correlation between a received sequence with a received and time-delayed sequence. The mobile station can also correct the frequency offset using the time repeating property of this sequence.
After acquiring the timing, the mobile station usually obtains the CP length in order to find the FFT or IFFT symbol boundary of the second OFDM symbol that the mobile station will decode. One conventional method to acquire the CP length information is to correlate CP 108 (FIG. 1) with the last portion of IFFT symbol 109 (FIG. 1) of the received OFDM symbols with all of hypotheses. However, this blind detection method is not very reliable.
Another method that has been disclosed to indicate the CP length in the timing reference sequence transmits one timing reference sequence from multiple possible timing reference sequences in the first acquisition pilot, TDM1 201. Each possible timing reference sequence typically indicates one possible CP length and generally has low correlation with the other timing reference sequences. When detecting the timing, the mobile station receiver correlates the received sequence with the other possible timing reference sequences and selects the one sequence (thereby the corresponding CP length) that yields the highest correlation. However, this method adds more hypotheses on the timing reference sequence, thereby increasing the complexity of the receiver and the probability of a false timing detection.
After detecting the CP length information, the mobile station may detect the PilotPhase or PilotPN in acquisition pilot TDM2 204 by first descrambling TDM2 204 sequence with a common descrambling sequence, and then correlating the descrambled TDM2 204 sequence with all possible TDM2 204 sequences.
After detecting TDM2 204, the mobile station similarly descrambles received TDM3 205 sequence with a unique descrambling sequence seeded with the PilotPhase or PilotPN value detected from TDM2 204. The mobile station then correlates the descrambled TDM3 205 sequence with all possible TDM3 205 sequences. If Walsh sequences are used as these sequences for TDM2 204 or TDM3 205, efficient correlation may be done with fast Hadamard transformation (FHT).
In a time division duplexing (TDD) system, the transmission times between different base stations are synchronous. In a frequency division duplexing (FDD) system, the transmission times between different base stations may be synchronous or asynchronous. Therefore, in acquisition pilot TDM3 205, the base station signals the synchronicity of system to the mobile stations using the Sync/Async bit so that the mobile stations can decode the control information accordingly. Other control information bits included in TDM3 205 are typically the Half-Duplex bit, Frequency-Reuse bit, and 4 least significant bits (LSBs) of the 9 or 12 bits system time to facilitate combining gain across multiple superframe preambles.
In some systems, the phases on TDM2 204 and TDM3 205 sequences are further shifted to one of three possible angles, according to a 3-state other sector interference (OSI) information scheme, to facilitate more effective control of the reverse link inter-cell interference. Once the mobile station acquires the system, it no longer needs to decode the static information (such as the Sync/Async bit) or the predictable information (such as PilotPhase or 4 LSBs of the system time) carried by TDM2 204 and TDM3 205. On the other hand, the OSI information is generally dynamic. It is not needed during the system acquisition stage, but can be helpful once the mobile station becomes active in the system. TDM2 204 sequences are rotated by 0, ⅔π, or 4/3π(i.e. same as −⅔π) according to an OSI value of “0”, “1”, or “2”, while TDM3 205 sequences are rotated by 0, −⅔π, or ⅔π according to the same OSI value “0”, “1”, or “2”. Therefore, the differential phase between TDM2 204 and TDM3 205 is 0, ⅔π, or −⅔π according to the same OSI value “0”, “1”, or “2”. This allows a simple non-coherent OSI detection scheme using the received TDM2 204 and TDM3 205 signals as the phase reference for each other without doing channel estimation, which can be very difficult for the neighboring sectors due to weak signals. The static or predictable information bits carried by TDM2 204 and TDM3 205 of the neighboring sectors may be obtained from the standard signaling messages, such as NeighborList Message and the like.
After detecting TDM3 205 during the power-up process, the mobile station can descramble received PBCCH 201 using the scrambling seed obtained from TDM2 204 and TDM3 205 and then decode the control information on PBCCH 201. Most of the control information carried on PBCCH 201 is static, except that the 9-bit or 12-bit system time, which is the index of the current superframe in the system, keeps increasing once every superframe and cycles through the 9-bit or 12-bit value. Therefore, after acquiring the system, the mobile station no longer needs to decode PBCCH 201, as the information in it is either static (such as the number of Guard tones) or predictable (such as the system time).
During the even-numbered superframes, the second to fifth OFDM symbols 202 in the preamble 200 are used for the SBCCH, which is used for broadcasting sufficient information, such as the information on the hopping patterns, the pilot structure, the control channel structure, the configuration of the transmit antennas, and the like, to enable the mobile station to demodulate traffic frames 102-103 (FIG. 1) that are transmitted by the base station.
There are several drawbacks in the existing preamble design as described above. First, there are too many hypotheses calculated in processing TDM1 timing reference sequences, which typically increases the complexity of receivers and the probability of false detection of timing. Second, if there is a false detection of timing or a detection error in the CP length information, the mobile will use the wrong received signal as the received TDM2 signals. After descrambling and FHT, the mobile station may still detect a valid FHT value based on this incorrect value, which would lead to error in the PilotPhase detection. The mobile station will then use the wrong PilotPhase to further descramble the received TDM3 signal, resulting in a wrong TDM3 detection also. Because the acquisition pilots TDM1, TDM2, and TDM3 do not have cyclic redundancy check (CRC) protection, the mobile station may not realize the detection error until descrambling the PBCCH, therefore unnecessarily prolonging the system acquisition time.